This invention broadly relates to grate devices for ore grinding or ore comminuting machines in commercial mining operations. More specifically, this invention is directed to an improved system for replacing and fastening grates to the rotatable interior of an ore grinding or ore comminuting machine.
Commercial mining operations require the use of ore comminuters or grinders, which reduce the size of large ore fragments for further refining. Several types of ore comminuters or grinders may be used, one of which takes the form of a large cylindrical rotatable shell that is rotated on a substantially horizontal axis and is driven by a very powerful motor through conventional reduction gearing. With this type of grinder, ore is introduced into one end of the drum through an inlet, and after reduction or comminution, the reduced ore is discharged through an outlet in the opposite end.
Within the drum, the charge of ore fragments rests at the bottom of the rotating drum. As the drum rotates, part of the ore charge is carried upwardly along the contoured inner surface of the drum until the carried fragments drop from the drum surface due to gravity, tumbling back onto the ore charge and breaking the fragments. This continuous process reduces the size of the fragments until they become small enough to pass through apertures in grate segments. These fragments are then discharged from the mill.
Individual grate segments are typically mounted on pulp lifters within the interior cylindrical surface of the rotatable drum. The individual grate segments are composed of a series of apertures with a predetermined size. Apertures in grate segments generally will range from 0.25xe2x80x3 to 4xe2x80x3(0.6-10.2 cm) depending on the specific application, with 1xe2x80x3 to 3xe2x80x3(2.5-7.6 cm) apertures being typical for most industry applications. An ore fragment must pass through at least one of the apertures before it is able to exit the mill. The grate segments are cast from alloys that are optimized to increase the wear rate while avoiding breakage caused by impacts from the ore charge and grinding media.
To maximize economic efficiency, ore comminuting mills of this type generally operate continuously, 24 hours a day. The ores being comminuted are highly abrasive. Therefore the continuous process wears the grate segments down over a period of time, depending on the type of ore and application. When grate apertures become worn, ore fragments that exit the mill may become larger than desired. When this happens, the grate segments must be replaced. It is desirable to replace the series of individual grate segments as quickly as possible because down time of the ore comminuting mill adversely affects the economic efficiency of the process.
Even when grate segments are not worn, it may be desirable to replace grates segments containing apertures of one size with grate segments containing apertures of a different size. This is especially important when the same ore grinder is used for variable ore types.
The process of replacing the series of individual grate segments presents certain problems that are not immediately evident. For example, the mere size of the equipment presents practical difficulties. A typical mill can measure 15 feet (4.6 m) long and over 28 feet (8.5 m) in diameter. Individual grate segments are commonly 2.5 to 4 inches (6.4-10.2 cm) thick, may be up to 4 feet (1.2 m) high and 6 feet (1.8) wide, and typically weigh up to several thousand pounds.
Individual grate segments that line the drum of the mill are conventionally fastened to pulp lifters on the cylindrical shell by transverse mounting bores that extend from the grinding surface to the mounting surface of the cylindrical drum. Each pulp lifter may include two such mounting bores. The cylindrical shell has the same number of mounting bores that are similarly spaced, permitting the mounting bores of the segments to be positioned in alignment. Once aligned, bolts are passed from the inside of the shell through the grate segments and the aligned mounting bores in the shell.
This type of segment fastening system works quite well in installing the individual grate segments. However, the bolt heads may be exposed at least partially to the comminution process, and by the time the grate segments require replacement, the bolt heads may be severely deformed. The continuous bombardment of fragments usually causes peening of the casting immediately around the bolt head, which may occlude the head and reduce its accessibility for removal.
Furthermore, there is at least some minimum space necessary between the sides and ends of adjacent grate segments to permit installation. During the ore comminution process, ore fines tend to fill up these spaces and are compacted in place. The grate segments may also be peened onto each other. This results in significant difficulty in removing the compacted grate segments when replacement is necessary. Even if the external nuts or the mounting bolts are removed relatively easily, this does not release the individual grate segments because of such compacting. Further, the bolts themselves have significant shear forces placed on them during the ore comminution process, often causing deformation to the point that they become skewed and tightly lodged within the interior of the rotating drum. The force necessary to remove a particular grate segment often requires the use of a crane and heavy hammering equipment.
Another approach that may be used instead of or in addition to forcibly hammering the bolts or grate segments is torch cutting the worn material from within the rotating drum. If the bolt head can be reached effectively by torch cutting, the bolt may be removed, thereby facilitating segment removal.
As will be appreciated, the conventional fastening of liner segments results in difficult grate segment removal when replacement is necessary, and this in turn causes significant mill down time.
The present invention is directed to a grate segment fastening system that can make grate segment installation and removal easier and less time consuming.
The present grate segment fastening system comprises a grate segment, and a grate holder. The grate segment is designed to work in conjunction with corresponding sides of the grate holder segments. For example, the grate segment generally may be configured with tabs on one end and mounting bores on the other end. The grate holder segments can be anchored onto existing pulp lifter sections on the interior of an ore grinder. The grate holder segments are typically anchored with threaded bolts or similar fastening means. In any case, the holder segments contain anchor bores that are configured to align with corresponding bores on the existing holder segments.
For assembly, in one example the grate segment is placed between adjacent holder segments so that the tabs interlock with recessed portions on one side of the grate holder. Once the tabs are securely in place, the grate is rotated until the mounting bores on the grate come into alignment with mounting bores on the other holder segment. Once the grate is in place, elongated members, such as bolts or pins, may be inserted through the mounting bores on the grate and into the mounting bores on the other grate holder segment. After the bolts or pins are inserted, protective plug members may be inserted over the bolts or pins to protect them from wear caused by the internal operation of the mill.
Preferably, the grate and grate holders are cast from a ferrous alloy such as pearlitic steel or martensetic white iron. In any case a suitable material can be chosen to maximize the wear-resistant characteristic of the grate holder while avoiding breakage due to brittleness.
In a typical milling scenario, the present invention would be used with existing mills to enable milling personnel to replace grate segments with less effort than present systems require. For example, preferred embodiments of the present invention only require the removal of two elongated members from the mounting bores on the grate. Then, the mounting end of the grate segment will rotate upward and away from the interior wall of the mill. At this point, the tabs on the grate easily can be slid outward from the recesses on the grate holder, thereby releasing the grate from the mill structure.
The grate holder segments then can be inspected visually to determine whether they need replacing. If they do not require replacement, a new grate segment easily can be installed by sliding its tabs into the recesses in the grate holder and aligning the mounting bores on the grate and grate holder. Elongated members are then inserted into the mounting bores, thereby locking the new grate in place. The holder can typically last through the effective wear life of at least two grate segments.